It is known that metals such as mercury, cadmium, and lead are accumulated in human bodies, which results in adverse effects on health. Therefore, it is important to analyze metals in biological samples such as urine and the like and samples of food and beverages such as water and the like.
In the analysis of metals, generally, foreign substances are removed from a sample as a pretreatment to remove a metal(s), and the separated metal(s) is analyzed. As the pretreatment, a solvent extraction is widely used. The solvent extraction is a method in which a metal in a sample is extracted into an organic medium by utilizing the polarity of a chelating agent to be bound to the metal according to the difference between the distribution coefficient of the metal in an aqueous medium and in the organic medium. The metal can be further concentrated by evaporating the organic medium from the extracted fraction. As a specific example of the solvent extraction, a dithizone method using, as the chelating agent, 1,5-diphenyl-3-thiocarbazone (hereinafter, also referred to as “dithizone”) that is insoluble in an aqueous medium under acidic conditions is described in JIS, for example (see Non Patent Literature 1 and Patent Literature 1). In the dithizone method, dithizone and a liquid sample are first mixed under acidic conditions to form a complex between the dithizone and a metal in the liquid sample in the mixture. Subsequently, an organic medium such as carbon tetrachloride or chloroform is added to the mixture. Accordingly, the complex is extracted in the organic medium because the distribution coefficient of the complex in the aqueous medium is different from that of the complex in the organic medium. By recovering this extracted fraction in the organic medium, the metal can be recovered from the liquid sample as a complex. The metal can be further concentrated by further evaporating the organic medium from the extracted fraction.